Glutamatergic Neurocircuitry Underlying MC4R Action Brain control of energy balance prevents obesity. An important component of this central regulation is the melanocortin system, which, working through melanocortin-4 receptors (MC4Rs), promotes weight loss. Indeed, absence of MC4Rs causes marked hyperphagia and massive obesity. Despite certainty regarding the importance of MC4Rs, there is a comparative lack of information regarding the underlying neurocircuitry. The goal of our studies is to understand the neural basis for MC4R-mediated regulation of energy balance. We have discovered that MC4Rs on both Sim1+ (likely the paraventricular nucleus - PVN) and Sim1- neurons (see below) control food intake. Of interest, MC4Rs on these two classes of neurons (Sim1+ and Sim1-) are functionally redundant, suggesting that they are interconnected. In a parallel set of studies, we have also discovered that the food intake- and body weight-regulating MC4Rs are located exclusively on glutamatergic (excitatory) neurons (marked by VGLUT2). Based upon these findings, and the work of others, we propose a novel, interconnected glutamatergic network to account for MC4R action. In this model, MC4Rs controlling food intake is on three groups of glutamatergic (excitatory) neurons. Two groups are Sim1-, are in the hindbrain, and constitute a linear, ascending, glutamatergic pathway that relays satiety signals from the gut to the forebrain (vagal afferents -> NTS -> lateral parabrachial nucleus (L-PBN)). The third group is Sim1+, is in the PVN, and sends descending, excitatory projections to the ascending pathway (at the NTS and L-PBN). AgRP and POMC neurons project to and engage MC4R-bearing glutamatergic neurons at each of these three sites. By placing MC4R-expressing neurons into an interconnecting pathway regulating satiety, this distributed model of melanocortin action accounts for the redundancy of MC4Rs on Sim1+ versus Sim1- neurons. Three Aims will probe this model. In Aim 1, we will test the underlying premises upon which the model is based (sufficiency versus necessity of MC4Rs on Sim1+ neurons, VGLUT2+ neurons, as well as on neurons in the PVN, NTS and L-PBN). In Aim 2, we will a) identify the key Sim1+/VGLUT2+ neurons within the PVN, b) use optogenetics to test connectivity (PVN -> NTS and PVN -> L-PBN), and then c) use DREADD technology to remotely, acutely and reversibly modulate function in vivo. In Aim 3, we will focus on the Sim1-/VGLUT2+ neurons in the NTS and L-PBN. In total, investigation of this distributed, interconnected, glutamatergic model of MC4R action should shed new light on neural circuits regulating food intake and energy balance.